I would tend to think "mods" for an insurer is anything that changes the original car driveline such as bigger tyres, lower suspension this kind of things. A shark antenna would not be a mod that would affect in anyway the driving experience in my book. But last time I renewed my insurance in july axa asked me if I had a performance intake filter...I dont, they must have a different view on mods. An air filter with higher air flow will mainly make your engine dirtier, nothing else. It has been established it does not do anything really to performance.

Now for radios, sound systems etc it is only worth mentionning if the insurance can provide an optional provision for accessories to cover these. Most of the time they dont. Ask them if you need to get covered.